Optimization of Cu(In,Ga)Se 2 formation by regulating the stacked metal layers structure-the role of metallic growth
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The absorber layers for chalcopyrite solar cells were fabricated by selenization of the stacked metal layers (SML). Co-sputtering and sequential sputtering methods were utilized to prepare the SML, and the variation of the stacking sequences and the effect of each stacked layer thickness were investigated. The stacking sequence of In/CuGa⋯In/CuGa was found having advantages in the SML growth and the average size of indium hillocks might be tailored by changing the thickness of each stacked layer. The SML in the stacking mode of In/CuGa⋯In/CuGa prepared while the thickness for each indium layer fixed at approximately 83 nm exhibited the desired morphology with evenly distributed indium hillocks in small diameters. The selenized CuInx Ga1x Se2 (CIGS) layer showed a smooth surface and largest grain size with phase segregation being suppressed effectively. The hole mobility of the best CIGS layers reached 8.36 cm2/V s.
I. INTRODUCTION
The chalcopyrite semiconductor CuInx Ga1x Se2 (CIGS) is considered as the most promising material for high efficiency thin film solar cells. Recently, the efficiency of CIGS solar cells has reached 21.7%.1 The absorber layers for high-efficiency CIGS solar cells are deposited by co-evaporation 2 in tradition, but the small areas of evaporation sources make uniformity for large films difficult to achieve, and the simultaneous control over four separate sources is complex. For this reason, a twostep method, selenization after sputtering stacked metal layers (SML) was applied.3 Magnetron sputtering is a preferred method for depositing many large-area, precise control needed films and in general has advantages in scalability to large areas without sacrificing uniformity.4 In spite of the fact that there are many reports about the two-step method,5–7 some problems still exist in it. One is that the selenized CIGS films were lacking in homogeneity due to the inhomogeneous sputtered metal layers8 which were often presented as indium hillocks on a smooth background. In addition, phase segregation was observed after selenization.9 To solve these problems, an optimized scalable method is needed in fabricating qualified SML for high efficiency CIGS solar cells. The morphology and structure of SML can be easily affected by the stacking methods, which also impact the selenized CIGS layers directly and hence Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.456
the cell’s performance. Herein SML for CIGS solar cells were deposited by co-sputtering and sequential sputtering, and the influence of different stacking sequences and single layer thickness to the SML and selenized CIGS layers were investigated in detail.
II. EXPERIMENTAL PROCEDURES
Prior to the deposition of SML, nearly 1 lm thick Mo thin films were deposited on soda-lime glass (SLG) substrates by three-step sputtering. SLG substrates were cleaned ultrasonically in acetone, dehydrated alcohol, and de-ionized water sequentially. The Mo films on SLG side and CIGS sid
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